In the traditional casting processes, a molten metal is poured into a mold and solidifies, or freezes, through a loss of heat to the mold. For relatively thermally insulating aggregate molds (such as those made from silica sand) this process is slow, significantly reducing the mechanical properties of the casting. While the rate can be increased by local metallic chill blocks placed in the mold, this is expensive and inconvenient on many molding lines. For this reason the casting of Aluminum (Al—) and Magnesium (Mg—) based alloys is often carried out in a permanent metal mold, with significant benefits to properties. The direction of freezing is, however, not easily controlled in either aggregate or permanent molds, so that shrinkage porosity remains a common fault for both types of castings, although one type has less porosity than the other.
Furthermore, the rate of heat extraction from all such molds is limited by the presence of the so-called ‘air gap’. This is the space that opens up between the cooling and contracting casting and the heating and expanding mold. The rate of transfer of heat from the casting is powerfully limited by this insulating layer of air. Regardless, these conventional casting processes extract or remove heat by way of surface cooling of the component, with the component being solidified either by the mold or by tooling.
When enough heat has been lost from the molten metal so that it has frozen, and cooled sufficiently to gain adequate strength so that it can support its own weight the resulting product, i.e., a casting, can then be removed from the mold. The separation of casting from mold can be somewhat energetic, often involving falling on to grids and/or tumbling for extracting the casting from green sand molds. For chemically bonded hard sands, mold removal usually requires a shake-out on a vibrating grid. Such processes are hot, dusty and noisy, involving the provision of clean and cool air for operators, significant dust extraction systems, and noise containment. For hard and strong castings of iron and steel such separation techniques are usually not damaging to the casting, but Al- and Mg-based alloys are relatively soft and easily damaged by such brutal mechanical techniques. These mechanical techniques can also cause distortion.
Following this initial separation, final cleaning, and possible removal of cores, still requires additional energetic processes such as shot blasting, or even significant energy in the form of heat treatment to burn out core residues.
Recently a novel approach to solve most of the above problems has been developed. It is called the ablation solidification process and is described in U.S. Pat. No. 7,216,691 the disclosure of which is incorporated herein by reference in its entirety.
“Ablation” is the term used in this application to refer to the removal of an aggregate mold by an erosion process in which the application of an ablating medium, such as a fluid causes the aggregate to disintegrate to grain size and the grains to be flushed away in the flow of the fluid. In this way, the surface of the solidifying metal component can be revealed, allowing direct contact between the ablating medium and the metal of the solidifying casting without the formation of any air gap. The direct contact maximizes heat flow from the metal, greatly increasing the rate of solidification and cooling of the metal. The timing of the application of the medium can be prior to complete freezing of the metal in the mold to maximize mechanical properties of the solidified metal, or can be delayed to minimize properties. An important specific example of ablation includes the use of an aggregate mold bonded with a soluble binder and the use of a solute, such as one containing water, as the ablating and cooling medium.
While the ablation process is a significant improvement over the known or conventional casting methods, it would be desirable to enhance the ablation process in order to provide higher productivity for metal products such as forgings, weldments and castings and enhance the properties of such products.
It would be particularly desirable to develop a process which would allow a single, unitary, ablated component or product formed from a molten metal (or perhaps even another type of material such as a plastic) to have different mechanical properties in various portions of the part. A component with one portion of the part having better mechanical properties or metallurgical properties than another portion of the same part has advantages in a variety of fields, including transportation, construction, manufacturing and the like.